1. Field of the Invention
This invention relates to an image forming apparatus such as a copying machine or a laser beam printer of an electrophotographic type, and a cartridge detachably attachable to the image forming apparatus.
2. Related Background Art
A known image forming apparatus using an electrophotographic recording process, for example, a laser beam printer, is provided with a photosensitive drum functioning as an image bearing member that is rotatably driven, a charging roller functioning as charging means for uniformly charging the surface of the photosensitive drum, a laser for exposing the surface of the photosensitive drum to light and forming an electrostatic latent image corresponding to an image signal, developing means for developing the electrostatic latent image with a toner and forming a visible image, a transferring roller for transferring the visible image (developer image) onto recording paper in the form of a sheet, fixing means for fixing the visible image transferred onto the recording paper, cleaning means, etc.
In this image forming apparatus, it is known to make the photosensitive drum and the charging roller integral with the cleaning means or the developing means and making them into a cartridge, and to make this cartridge (hereinafter referred to as the process cartridge) detachably attachable to the image forming apparatus to thereby realize a maintenance-free image forming apparatus.
In such an image forming apparatus, when for example, the functions of the constituent parts incorporated in the process cartridge are lowered by a long period of use, the entire process cartridge is interchanged.
This interchanging work (process) is a very simple process of opening the main body of the image forming apparatus by one touch, taking out the old process cartridge from the interior of the main body of the image forming apparatus, and mounting an unused new process cartridge on the main body of the image forming apparatus, and can be carried out easily by an operator himself.
The life (interchange time) of this process cartridge is determined chiefly by the abrasion of the photosensitive drum and the developing roller and the consumption of the toner. The abrasion of the photosensitive drum and the developing roller can be schematically calculated from their total number of revolutions. The life (interchange time) can be calculated from the total number of printed sheets proportional to the total number of revolutions. Also, the consumption of the toner can be detected by toner remaining amount detecting means.
The amount of remaining toner can be detected each time. The process cartridge, however, can be arbitrarily interchanged by the user and therefore, it is desirable that the total number of printed sheets regarding the life of the photosensitive drum and the developing roller be kept in custody in the process cartridge.
For example, there is known a method of storing life information in a memory mounted on the process cartridge.
On the other hand, even in image forming apparatuses having different features (different kinds of apparatuses), when only the number of sheets printable within a minute differs, the process cartridges are sometimes made common. If the process cartridge is changed each time an image forming apparatus is put on sale as a new product, an increase in the cost of production and further, a place of custody for the process cartridge in connection with sale will become necessary for each type of machine. So, it is desirable that the process cartridges, which are expendibles, be made common as far as possible.
Also, there are generally two methods of making the number of sheets printable within a minute different. To increase the number of printed sheets per unit time, there are a method of increasing the rotating speed itself of the photosensitive drum, and a method of narrowing the interval between sheets undergoing continuous printing. This interval between the sheets will hereinafter simply be called the inter-sheet.
When the rotating speed of the photosensitive drum is increased, this main body does not differ in the relation between the total number of revolutions and the total number of printed sheets from a main body having its rotating speed kept as it is. This is because the number of printed sheets increases in proportion to an increase in the number of revolutions per unit time.
Accordingly, if the total number of printed sheets concerned with the total number of revolutions is stored in a memory, the life (interchange time) of the photosensitive drum, the developing roller, etc., can be correctly calculated even by a conventional calculating method, even between main bodies differing in printing speeds from each other.
On the other hand, in a method of shortening the inter-sheet, the rotating speed of the photosensitive drum is equal between the respective main bodies, and the conveying speed of the sheet is also equal between the respective main bodies. Accordingly, various conditions concerning an image, such as a laser applying condition and a fixing condition can be made constant and therefore, not only the image forming apparatus can be developed within a short period, but also various parts can be made common and therefore, the cost of the apparatus and the reliability of the parts can be improved.
However, when the inter-sheet is shortened to thereby make the number of printed sheets per unit time (a minute) different, the relation between the total number of revolutions of the photosensitive drum, the developing drum, etc., and the total number of printed sheets becomes different between the main bodies of image forming apparatuses having different features. In the main body wherein the inter-sheet is narrowed, as compared with the main body in which the inter-sheet is kept as it is, the rate of the time of the inter-sheet during which an image is not printed becomes short. That is, the ratio of the time contributing to printing increases and therefore, in the main body in which the inter-sheet is short relative to the same total number of revolutions, the total number of printed sheets becomes greater.
Accordingly, when the life (interchange time) of the photosensitive drum and the developing roller is to be calculated, the conventional calculating method is not successful between the main bodies differing in the inter-sheet from each other.
So, there is also a method of uniformly adjusting the setting of the threshold value of the total number of printed sheets to the main body in which the inter-sheet is longer. However, in spite of the ability of a main body in which the inter-sheet is shortened to print more, the cartridge's life must be shortened in conformity to the main body whose inter-sheet is long, since the cartridge can be used in main bodies whose inter-sheet is short or long, and is cannot be said to be an effective method. Particularly, when the photosensitive drum is abraded and exceeds its life, it is desirable to turn on the end of life lamp of the image forming apparatus, once stop printing, and call upon the user to interchange the process cartridge. The reason why the main body is once stopped by the life of the photosensitive drum is for preventing the occurrence of such a faulty image as will produce damage to the main body, such as twining or a jam in the fixing device due to the photosensitive drum having exceeded its life.
Also, in some cases, the user employs two kinds of main bodies, i.e., a main body in which the inter-sheet is short and a main body in which the inter-sheet is long. If the process cartridge has interchangeability, there is the possibility of the same process cartridge being alternately mounted on the two main bodies. In this case, there may occur the absurdity that the process cartridge reaches the end of its life in one main body and does not reach the end of its life in the other main body.